Process and apparatus for absorption of zinc vapour in molten lead

ABSTRACT

A process and apparatus for absorbing zinc vapour in a molten lead is characterized in that a gas containing zinc vapour is contacted with and then separated form a flowing stream of molten lead in a cyclone.

This invention relates to an improved apparatus and process for absorbing zinc vapour into molten lead.

Gases containing zinc vapour are commonly generated in zinc smelting processes (for example, the Imperial Smelting Process (ISP)) in slag fuming; and in the treatment of zinc-containing dusts and residues.

Existing industrial processes for recovering zinc from gases containing zinc vapour are essentially of three kinds, of which the abovementioned ISP process is one. The ISF process uses rotors or impellers to splash lead from a molten pool into the zinc-laden gas stream. In an alternative ISP process, zinc is used as the condensing medium rather than lead. The so-called SKF process uses molten lead or molten zinc in the form of a spray or curtain as cooling metal or medium towards which the gas stream containing zinc vapour is directed.

References relevant to the processes mentioned include D Temple, "Zinc-lead blast furnace--key developments", 1980 extractive metallurgy lecture to AIME, Metallurgical Transactions B vol. 2B, pp 343-352; GB 1,010,436 (Imperial Smelting); and GB 2,122,648 (SKF).

The ISP process suffers from accretions at the mouth of the condenser and in the condenser/absorption chamber causing frequent stoppages of the furnace operation. These accretions form on surfaces that are below the temperature where solid ZnO forms by the reaction Zn+CO₂ →ZnO+CO. This reaction is called the reversion reaction and the temperature at which it occurs the `reversion temperature`. The SKF process is free of this problem only because of the highly reduced gas entering the condenser.

Both processes suffer from shortcomings such as listed below:

Build-up of dross within the condenser/absorber

Poor efficiency

The need for a large cooling and liquation circuit which is expensive to build and maintain

Large carryover of lead droplets in the off-gas stream leading to lower zinc recoveries.

Further shortcomings of the existing process technologies and how they are overcome by the present invention will be described below.

In a principal aspect the invention provides a process for absorbing zinc vapour in molten lead characterised in that a gas containing zinc vapour is contacted with and then separated from a flowing stream of molten lead in a cyclone.

Preferably the cyclone contact stage is preceded by a stage in which the molten lead is introduced into and contacted with the stream of gas containing zinc vapour in a mixing chamber.

In a more preferred embodiment, the apparatus of the invention comprises a refractory lined crossover or off-take with an outlet at the bottom which opens into the mixing chamber that joins a cyclone.

Lead is introduced into the chamber by a lead spray directed into the gas stream. This spray produces a dispersion of lead droplets within the gas stream. Lead may be also introduced by additional sprays that completely wet the walls of both the vertical section before the cyclone and also within the cyclone itself.

The vertical chamber before the cyclone may also house one or more banks of static mixing elements. These elements not only serve to mix the lead droplets and gas together but also break up the lead droplets. This action causes a high degree of shear and a large contacting area as well as turbulence in both phases.

Mixing columns housing static mixing elements are known, and reference may be made by way of example to one such apparatus described in U.S. Pat. No. 4,744,928 to Sulzer Brothers Limited of Switzerland. The static mixing elements in that design are disposed within the chamber in a manner that deflects the flow of fluid impinging thereon and thereby promotes efficient mixing of gases and/or liquids passing through the chamber.

Provided the static mixing elements promote efficient mixing, their precise configuration is not critical to the present invention.

The molten lead now containing the absorbed zinc is passed into a system for recovery of the latter as well as for recirculation of the molten lead for renewed absorption. The off-gas is passed to a conventional gas cleaning system.

In the accompanying drawings:

FIG. 1 is a diagrammatic representation of an exemplary preferred embodiment of the process according to the invention;

FIG. 1a is a diagrammatic representation of an alternative exemplary preferred embodiment of the process according to the invention; and

FIG. 2 is a more detailed illustration of a preferred form of the components 2 (in part), 3 and 4 of FIG. 1.

The construction and operation of the apparatus will be better understood by reference to FIG. 1 of the accompanying drawing. In this FIG. 1 represents zinc-laden gas from the smelting or slag fuming operation, and 2 represents a refractory lined off-take. A burner 3, called the `transition burner`, is provided to maintain the temperature of the lower region of the refractory above the Zn--ZnO reversion temperature. A lead spray (alternatively, a bank of lead sprays) 4 direct(s) molten lead to a vertical chamber 5 which may contain mixing elements 5a to enhance the contact between the zinc-laden gas and the molten lead.

A cyclone 6 serves both to contact and to separate the gas and the lead, the latter passing to the gas-cleaning system 7. The zinc-rich lead stream 8 is passed to the pump sump 9 provided with pump 10 conveying lead back through the absorption system. A zinc-lead stream is pumped from the sump 9 via line 11 to a lead cooler 12 and returned to the sump, and via line 11a to lead sprays 4. Numerals 13 and 14 represent a cooling water inlet and outlet respectively. A small portion of the zinc-lead is passed via line 16 to the liquation pot 17, which is provided with cooling water inlet 18 and outlet 19 respectively. Numeral 20 represents the zinc product and 21 a launder for returning liquated lead to the pump sum 9.

Turning to FIG. 1a, line 16 of FIG. 1 is replaced by line 16a from pump 10 direct to liquation pot 17 and bypassing lead cooler 12.

The items illustrated in FIG. 2 provide an example of a preferred transition burner and lead spray assembly.

Process gas, indicated by numeral 39, enters at the top and flows downward through the assembly.

A fuel such as propane is precombusted with oxygen. The hot gas is introduced tangentially into a toroid 30 penetrating a circumferential offtake body 38. The toroid 30 serves two purposes. Firstly, it evenly distributes the gas before it exits the burner and, secondly, it serves to heat the offtake body 38.

Numeral 30a indicates an exit port for hot gas into the central open space defined by the offtake body 38, the gas exiting as shown by arrow 33.

Upper and lower circumferential mains, 31 and 32 respectively, are shown for supply of streams of lead or zinc-lead in streams indicated by arrows 34 and 35 respectively. Numeral 36 indicates the presence of baffles to remove the swirl from stream 34 before it is deflected downwardly and towards the centre of the open space.

A circumferential truncated cone 37 extends downwardly into the stream of gas 39 and forms part of exit ports 30a.

The surface temperature of the inside of the toroid 30 is maintained at around 1500° C. Furthermore, the burner is run to give a good exit gas velocity (18 m/s) of highly reducing gas (CO/CO₂ =10). As a fine control on the temperature, and to achieve good velocities without the burner getting too hot, nitrogen is also introduced into the burner.

The offtake body 38 is heated by the burner otherwise its surface would fall below the reversion temperature. The lower part of the offtake body 38 is directly above the region where lead is sprayed into the absorber. Consequently this lower part loses heat by radiation to the lead.

The gases exiting from the exit port 30a serve primarily to stop zinc from diffusing to the top lip of the lead spray causing an accretion. This top lip will always be held below the reversion temperature because of the lead in the spray.

The shape of cone 37 was found to be necessary to give protection against diffusion of process gas onto the cold lip of the lead spray. The high turbulence of the process gas greatly enhances the possibility of diffusion.

The top lead spray 34 is designed to introduce lead to the centre of the process gas stream. Lead may be introduced tangentially into a main 31 surrounding the spray. The swirl introduced to the lead by the tangential inlet is removed by baffles so that the lead is introduced radially but inclined downwardly into the process gas stream.

Lead or zinc-lead is introduced tangentially into a main 32 surrounding the spray. The lead maintains its high swirl and as it exits the spray it flattens itself against the walls. The swirl is sufficient to give a uniform coating down the mixer column.

As well as introducing lead or zinc-lead to the centre of the gas stream the top spray 34 is needed to contain the highly swirled bottom spray 35. Without this containment, lead from the bottom spray would flush upwards.

The spray system is designed so that splash upwards onto the refractory areas or upward movement from the bottom spray is substantially non-existent. If splash or upward movement occurs, the refractory is cooled below its reversion temperature and accretion forms.

The outlets of the top and bottom sprays are designed to be close together so that there are no unwetted areas of steelwork.

The principal benefits achievable by preferred embodiments of the present invention (designated `Pasminco`) are demonstrated vis-a-vis the characteristics of existing technologies in Table 1 below.

    __________________________________________________________________________     Characteristic                                                                         Pasminco SKF   ISP    Reasons                                          __________________________________________________________________________     Contactor size                                                                         Of the order of                                                                         Large Large  Pasminco design of the absorber                          50% reduction in      provides for much more shear and                         volume over ISP,      turbulent contact between gas and                        30% over SKF          liquid.                                          Settler size                                                                           Of the order of                                                                          --    --    Pasminco settler uses a cyclone 6                        75% reduction         to separate the lead droplets out                        over others           of the gas, the other technologies                                             rely on gravity settling.                        Liquation/                                                                             50% reduction in                                                                        Large Large  Pasminco design separates the                    cooling size                                                                           plant floor area      liquation/cooling functions into                                               two different devices. The                                                     Pasminco cooler treats only highly                                             turbulent, high temperature zinc-                                              lead while the liquation pot                                                   treats only a small proportion of                                              the zinc-lead in a quiescent bath.                                             The ISP design relies on a                                                     quiescent open launder where all                                               the lead is brought to the                                                     liquation temperature.                           Growth of                                                                              Eliminated                                                                              N.A. due                                                                             Frequent                                                                              The transition burner on the                     curtain          to high                                                                              cause of                                                                              Pasminco design keeps the entry to               accretions at    CO    shutdowns                                                                             the condenser above the reversion                the condenser/   content      temperature. The transition                      absorber inlet                region between the hot and cold                                                zones is entirely occupied by a                                                flame or a lead spray making                                                   reversion impossible. In the SKF                                               design reversion would occur here                                              if the gas mixture was subject to                                              reversion. In the ISP design the                                               curtain forms in this area.                      Build up or                                                                            Eliminated                                                                              Severe                                                                               Severe Both SKF and ISP rely on dross                   dross within the build-ups                                                                            build-ups                                                                             removal by dross being sucked down               condenser/       cause cause  with the lead into the pump sump.                absorber         shutdowns                                                                            shutdowns                                                                             This is not very positive and                                                  dross builds up. Pasminco has no                                               internal lead pool and dross must                                              exit with the lead.                              Zinc recovery                                                                          Of the order of                                                                         80-90%                                                                               90%    The more efficient contacting of                 efficiency                                                                             95% or better         the Pasminco device produces a                                                 higher recovery for the same                                                   reasons that permit a smaller size                                             device. The lower amount of dross                                              make and lower lead droplet                                                    carryover both contribute to                                                   increased efficiency.                            Lead droplet                                                                           Very Low High  High   The cyclone action of Pasminco's                 carryover                     design elimiates the lead droplet                                              carryover. The other technologies                                              rely on gravity separation.                      Furnace dust                                                                           Some     Nil   Nil    The cyclone action of Pasminco's                 collection                    design also collects some of the                                               furnace dust.                                    Weight  Low      Medium                                                                               High - Pasminco does not have an internal                                      500 t  lead pool, neither does it                                                     incorporate a large launder to                                                 cool the lead.                                   Power   Low      Low   High - The ISP design as a high power                   Consumption            700 hp consumption due to the power                                                   required to turn the rotors.                     __________________________________________________________________________

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to above. 

I claim:
 1. A process for absorbing zinc vapour in molten lead which process comprises the steps of:causing a stream of gas containing zinc vapour to enter a mixing chamber; spraying molten lead into the stream of gas containing zinc vapour to form a dispersion of molten lead droplets in the stream thereby contacting the molten lead droplets with the zinc vapour and causing the lead droplets to absorb the zinc vapour; and passing the stream of gas containing zinc vapour having droplets of lead dispersed therein into a cyclone which serves both to contact and to separate the gas and the lead.
 2. A process as claimed in claim 1, further comprising the steps of spraying molten lead onto the walls of the mixing chamber to prevent reversion on the cold interior surfaces of the chamber.
 3. A process as claimed in claim 1, further comprising heating refractory surfaces in the general region of the mixing chamber wherein lead is introduced, to a temperature above the Zn/ZnO reversion temperature.
 4. A process in claimed in claim 1, further comprising spraying molten lead through first spraying means having an upper and lower lip and inserting a buffer layer of hot reducing gas between the gas containing zinc vapour and the upper lip of the spraying means.
 5. A process according to claim 2 comprising spraying molten lead onto the walls of the mixing chamber by means of a tangentially directed spray, and spraying molten lead into the steam of gas containing zinc vapour by means of a radially directed spray located adjacent to but upstream of the tangentially directed spray, such that the tangentially directed spray is prevented from the contacting refractory surfaces upstream of the sprays.
 6. A process as claimed in claim 1, further comprising providing one or more banks of static mixing elements in the mixing chamber to promote mixing between the zinc vapour and the molten lead.
 7. The process as claimed in claim 1, further comprising passing a zinc rich lead stream from the cyclone to a reservoir of zinc/lead, cooling part of the zinc/lead, and recycling part of the zinc/lead to the mixing chamber.
 8. A process as claimed in claim 7, further comprising withdrawing a portion of the cooled stream to a zinc separation stage.
 9. A process as claimed in claim 8, wherein the portion of the cooled stream withdrawn to the zinc separation stage is a relatively small proportion of the mass flow of zinc/lead in the circuit.
 10. A process as claimed in claim 7, further comprising withdrawing a portion of zinc/lead from the reservoir and transferring the withdrawn portion without cooling to a zinc separation stage.
 11. A process as claimed in claim 8, wherein the zinc separation stage comprises liquation means.
 12. A method according to claim 1 wherein the stream of gas containing zinc vapor having droplets of lead disposed therein is passed downwardly in said mixing chamber.
 13. A method according to claim 12 wherein said cyclone is oriented substantially horizontally and wherein the gas stream which is passed downwardly enters said cyclone tangentially.
 14. A method according to claim 4 further comprising generating said hot reducing gas in a burner.
 15. A method according to claim 14 wherein said reducing gas is highly reducing.
 16. Apparatus for absorbing zinc vapour in molten lead which comprises a mixing chamber and a cyclone in communication with the mixing chamber, the mixing chamber having inlet means for receiving a stream of gas containing zinc vapour, first spraying means for spraying molten lead into the stream of gas containing zinc vapor, and second spraying means for spraying molten lead onto internal walls of the mixing chamber, the cyclone being positioned to receive from the mixing chamber, in use of the apparatus, a stream of gas containing zinc vapour and droplets of molten lead dispersed therein.
 17. Apparatus as claimed in claim 16, wherein the inlet comprises a refractory-lined conduit having an outlet that opens into the mixing chamber.
 18. Apparatus as claimed in claim 17, further comprising a transition burner for maintaining, in use of the apparatus, the temperature of the refractory-lined conduit above the Zn/ZnO transition temperature and for inserting, in use of the apparatus, a layer of heat reducing gas between the gas containing zinc vapor and an upstream portion of the first spraying means.
 19. Apparatus as claimed in claim 16, further comprising means for conveying a zinc-rich lead stream from said cyclone, means for recovering zinc from the zinc lead stream, and means for returning a portion of the zinc/lead to the mixing chamber.
 20. Apparatus according to claim 19, which includes means to cool a portion of the zinc/lead.
 21. Apparatus according to claim 19 further comprising means to convey said zinc-rich lead stream to a sump for zinc/lead, means to return a portion of the zinc/lead to the mixing chamber, means to convey a portion of the zinc/lead to cooling means, means to convey a portion of the zinc/lead to a liquation pot, and means to return zinc depleted lead from said liquation pot to said sump.
 22. Apparatus according to claim 21 further comprising means to return cooled lead/zinc to the sump.
 23. Apparatus according to claim 16, wherein the first spraying means is located downstream from the refractory-lined conduit and upstream from the second spraying means, the first spraying means being adapted to spray molten lead radially into said mixing chamber.
 24. Apparatus according to claim 16, wherein, in use of the apparatus, the mixing chamber is oriented substantially vertically and the cyclone is oriented substantially horizontally. 